In an open study 17 women with confirmed, severe and long-standing premenstrual syndrome used photic stimulation with a flickering red light, every day for up to four menstrual cycles. At the end of treatment prospectively recorded median luteal symptom scores were reduced by 76% (95% confidence interval 54-93, P < 0.001), with clinically and statistically significant reductions for depression, anxiety, affective lability, irritability, poor concentration, fatigue, food cravings, bloating and breast pain. Twelve of the 17 patients (71%) no longer had the premenstrual syndrome. One patient failed to improve. One patient withdrew because of worsening premenstrual depression, but photic stimulation was otherwise well tolerated. The improvement is greater than that reported for relaxation or in open studies of fluoxetine, and much more than historical placebo rates. Photic stimulation may be a useful treatment for the premenstrual syndrome, and by its suggested action on circadian rhythms may have wider therapeutic applications.